The present invention relates to a method and an apparatus for forming a coil spring, more particularly a barrel coil spring wherein each end has consecutive coils with decreasing coil diameters.
Methods and apparatus for hot forming coil springs are well known in the art. Typically, such apparatus comprises a rotatable mandrel about which is wound a heated steel rod. Means are provided to clamp one end of the steel rod onto the mandrel and, as the mandrel is rotated, guide means serve to guide the rod as the mandrel traverses along its longitudinal axis to form the coil spring. Once formed, the mandrel may be withdrawn and the coil spring removed from the apparatus.
Such devices have proven very efficient for the manufacture of coil springs having substantially uniform coil diameters. These devices may also be utilized to form coil springs having one barrel end wherein the consecutive coils adjacent this end have decreasing coil diameters. To accomplish this, the rotating mandrel may be formed with a reduced diameter end portion so as to form the coils having decreasing diameters near the end of the coil spring. However, since the length of the mandrel is greater than that of the coil spring and since the mandrel must be withdrawn longitudinally from within the coil spring after the completion of the forming process, it is not possible to form a barrel shaped coil spring, wherein both ends have coils with decreasing diameters, utilizing this apparatus. Thus, it is necessary to use a second forming device to form the second barrel end on the coil spring.
Typical of such devices are those shown in U.S. Pat. Nos. 4,424,695 and 4,571,973. In these arrangements, a shaping member or winding jig is inserted into the coil spring near the second end which is to be reshaped from that having a generally uniform coil diameter to one having consecutively decreasing diameters, and the end of the spring is attached to a rotatable head or spindle. The spring is clamped to the device and the rotatable head or spindle is rotated so as to form the second barrel end. Subsequently the shaping member or winding jig is withdrawn and the spring is unclamped and removed from the device.
While these devices have been generally successful, the apparatus involved has proven to be extremely complex resulting in relatively high manufacturing costs and inherently decreasing the reliability of the appratus. The complexity of the device is increased due to the necessity of having the shaping member inserted into the coil undergo both radial and longitudinal motion to be properly positioned within the coil spring, or to provide the rotary spindle with both rotational and laterally transverse movement capabilities. The number of coil turns that may be reduced in diameter is somewhat limited due to the positioning of the shaping member or winding jig and also since only the roll in head or the rotary spindle can provide the requisite rotation to the end of the coil spring.